1. Field of the Invention
The present invention relates to a carbon dioxide absorbent, and more particularly, to an alkali carbonate-based carbon dioxide absorbent containing an added sterically hindered cyclic amine that may improve a rate of carbon dioxide absorption, reduce an amount of renewable energy used, and prevent salt production and phase separation, and a method for removing carbon dioxide using the same.
2. Discussion of Related Art
Carbon dioxide, one of six greenhouse gases which contribute to global warming, is an acidic gas that is emitted in large quantities by a plethora of stationary sources. To separate and remove carbon dioxide emissions that may be attributed to energy industries, many studies have been conducted on economically efficient processes for absorbing carbon dioxide through use of an aqueous absorbent.
The most widely used absorption process includes using an aqueous absorbent alkanolamine process (for example, monoethanolamine, diethanolamine, triethanolamine, and the like) and the Benfield process using potassium carbonate may be mentioned.
The alkanolamine process uses 20 to 30 wt % of a solution comprising water and various alkanolamines used to absorb carbon dioxide, and because of rapid carbon dioxide absorption, has been in commercial use since the nineteen seventies.
However, this process has issues of using an excessively high amount of energy in a range of 4.0 to 4.2 GJ/ton CO2, (in the case of, for example, monoethanolamine (MEA)) during regeneration, and corrosion of components made from carbon steel by oxidation reactions of pollutants present in combustion exhaust, such as, for example, O2, SO2, and NOx, and the like.
The most pressing issue of all is high renewable heat. In order to solve this issue, absorbents having steric hindrance have been developed in which steric hindrance occurs by attaching a methyl or ethyl group to a nitrogen atom in an alkanolamine used to remove carbon dioxide via carbamate bonds in order to reduce the bond strength. A representative example is 2-amino-2-methyl-1-propanol (AMP).
This absorbent absorbs carbon dioxide less rapidly than MEA, but has an advantage of using a considerably low amount of renewable energy. Similarly, an absorbent KS-1™ of Mitsubishi Heavy Industries, Ltd. known for using a sterically hindered amine as a major constituent exhibits usage of a very low amount of renewable energy in a range of 2.8 to 3.2 GJ/ton CO2. However, the sterically hindered amine used is mostly alkanolamines, in particular, primary amines with sterically hindering groups.
Advanced carbon capture technologies using sterically hindered amines have been suggested. For example, Japanese Patent Application No. 1994-242915 (registered as Japanese Patent No. 3197173, Jun. 8, 2001) of Kansai Electric Power Co., Inc. and Mitsubishi Heavy Industries, Ltd. discloses a method for removing carbon dioxide in combustion exhaust. This art teaches a compound represented by the following chemical formula as a major constituent of an absorbent to improve the CO2, absorption capacity per unit mole and per unit volume of the absorbent, increase the absorption rate, and reduce an amount of renewable energy used. In the following chemical formula, R1 is a lower alkyl group and R2 is hydrogen or a lower alkyl group. Also, this art claims a system containing a sterically resistant piperazine derivative in an aqueous solution wherein the concentration of the piperazine derivative in the aqueous solution is 15 to 65 wt %.

However, when the concentration is 20 or more wt %, an additional material, such as, for example, an alcohol, is required to solubilize the piperazine derivative. Also, this system is merely a binary system in which two constituents, that is, water and the piperazine derivative are simply mixed. Due to an absence of alkali carbonate that tends to promote formation of bicarbonate, this art is unfavorable in that a bicarbonate forming reaction is not dominant over a carbamate forming reaction during CO/absorption reaction.
To capture carbon dioxide that is an acidic gas, processes using liquid-type alkalis, such as, for example, NaOH, Na2CO3, K2CO3, KOH, and the like exist. In this instance, the Benfield process and the Catacarb process may be mentioned as representative examples. The Benfield process adds alkanolamines, such as, for example, mainly diethanolamine (DEA) to improve the reaction rate. The Catacarb process uses potassium carbonate as a major constituent and unknown organics or inorganics as a promoter to improve the reaction rate.
However, because these processes must operate an absorber and a stripper at 120° C. or more under the inlet gas pressure of 10 atmospheres (atm) or more to prevent the production of a salt of potassium bicarbonate, they have a drawback of using a large amount of energy.
Recently, an improved process using potassium carbonate was published in WO 2004/089512 A1 by Dr. Rochelle et al, The University of Texas.
According to this art, an absorbent contains potassium carbonate and piperazine or a piperazine derivative. Here, the piperazine or piperazine derivative serves to improve the absorption rate. Since piperazine is an alkylamine of a cyclic diamine structure and has two amino groups, the absorbent containing potassium carbonate and piperazine can efficiently absorb carbon dioxide and reduce the amount of renewable energy used.
However, it is predicted that this absorbent will have a problem in solubility. Afterwards, Korean Patent Application No. 2005-7018813 discloses the use of piperazine to prevent the salt production and improve the absorption rate, however piperazine is not sterically hindered.
As described in the foregoing, conventional arts using potassium carbonate, as a benefit of potassium carbonate, efficiently use a low amount of renewable energy, and to overcome the drawbacks of low absorption rate and salt production that are caused by the use of potassium carbonate, use a non-sterically hindered piperazine derivative as an additive. However, a verification experiment conducted using the corresponding conditions reveals that a salt is produced and chemical phase separation frequently occurs under certain conditions.